Progenitors Of Galaxies Research Articles

Spatially resolved spectroscopy of early-type galaxies over a range in mass

Long-slit spectra have been obtained with the Keck telescope for a sample of 11 early-type galaxies covering a wide range in luminosity and hence mass. Rotation velocity and velocity dispersions, together with 19 Lick line-strength gradients have been measured, to, on average, two effective radii. Stellar population models taking into account the effect of the non-solar chemical composition have been used to derive ages, metallicities and α/Fe abundances along the radius. We find that line-strength gradients are due mainly to variations of the total metallicity with the radius. One galaxy out of 11 shows very strong age gradients, with a young central component, while the age gradient for the rest of the sample is very shallow or consistent with zero. We also find small variations in the [α/Fe] ratio with radius. Contrary to what is expected in simple collapse models, galaxies show both positive and negative [α/Fe] profiles. This rules out a solely inside-out, or outside-in, formation mechanism for all early-type galaxies. We do not find a correlation between the metallicity and the [α/Fe] gradients, and the local metallicity is not correlated with the local velocity dispersion for all the galaxies of our sample, which rules out scenarios where the delay in the onset of the galactic winds is the only mechanism producing the metallicity gradients. We found that metallicity gradients are correlated with the shape of the isophotes and the central mean age and metallicity of the galaxies, for galaxies younger than ∼10 Gyr. We show that the correlation between the gradients and the central values is not due to the correlation of the errors and indicates that the same process that shaped the gradient, also modified the structural parameters of the galaxies and triggered star formation in their centres. This strongly supports the merger scenario for the formation of these systems, where the degree of dissipation during those mergers increases as the mass of the progenitor galaxies decreases. Finally, we also find a dichotomy in the plane grad [α/Fe]–[α/Fe] between galaxies with velocity dispersions below and above ∼200 km s−1, which requires confirmation with larger samples.

The build-up of the colour-magnitude relation in galaxy clusters since z 0.8

Using galaxy clusters from the ESO Distant Cluster Survey, we study how the distribution of galaxies along the colour-magnitude relation has evolved since z~0.8. While red-sequence galaxies in all these clusters are well described by an old, passively evolving population, we confirm our previous finding of a significant evolution in their luminosity distribution as a function of redshift. When compared to galaxy clusters in the local Universe, the high redshift EDisCS clusters exhibit a significant deficit of faint red galaxies. Combining clusters in three different redshift bins, and defining as `faint' all galaxies in the range 0.4 > L/L* > 0.1, we find a clear decrease in the luminous-to-faint ratio of red galaxies from z~0.8 to z~0.4. The amount of such a decrease appears to be in qualitative agreement with predictions of a model where the blue bright galaxies that populate the colour-magnitude diagram of high redshift clusters, have their star formation suppressed by the hostile cluster environment. Although model results need to be interpreted with caution, our findings clearly indicate that the red-sequence population of high-redshift clusters does not contain all progenitors of nearby red-sequence cluster galaxies. A significant fraction of these must have moved onto the red-sequence below z~0.8.

The Stellar Masses and Star Formation Histories of Galaxies atz≈ 6: Constraints fromSpitzerObservations in the Great Observatories Origins Deep Survey

Using the deep Spitzer Infrared Array Camera (IRAC) observations of the Great Observatories Origins Deep Survey (GOODS), we study the stellar masses and star formation histories of galaxies at z approx 6 based on the i_(775)-band dropout sample selected from the GOODS fields. In total, we derive stellar masses for 53 i_(775)-band dropouts that have robust IRAC detections. These galaxies have typical stellar masses of ~10^(10) M_⊙ and typical ages of a couple of hundred million years, consistent with earlier results based on a smaller sample of z ≈ 6 galaxies. The existence of such massive galaxies at z ≈ 6 can be explained by at least one set of N-body simulations of the hierarchical paradigm. We also study 79 i_(775)-band dropouts that are invisible in the IRAC data and find that they are typically less massive by a factor of 10. These galaxies are much bluer than those detected by the IRAC, indicating that their luminosities are dominated by stellar populations with ages ≾ 40 Myr. Based on our mass estimates, we derive a lower limit to the global stellar mass density at z ≈ 6, which is 1.1-6.7 × 10^6 M_⊙ Mpc^(-3). The prospect of detecting the progenitors of the most massive galaxies at yet higher redshifts is explored. We also investigate the implication of our results for reionization and find that the progenitors of the galaxies comparable to those in our sample, even in the most optimized (probably unrealistic) scenario, cannot sustain the reionization for a period longer than ~2 Myr. Thus most of the photons required for reionization must have been provided by other sources, such as the progenitors of the dwarf galaxies that are far below our current detection capability.

What Triggers Galaxy Transformations? The Environments of Poststarburst Galaxies

There are good observational reasons to believe that the progenitors of these red galaxies have undergone starbursts, followed by a poststarburst phase. Poststarburst (K+A or E+A) galaxies appear in the SDSS visible spectroscopic data by showing an excess of A-star light but relatively little Hα emission. We investigate the environments of these galaxies by measuring (1) number densities in 8 h-1 Mpc radius comoving spheres, (2) transverse distances to nearest Virgo-like galaxy clusters, and (3) transverse distances to nearest luminous-galaxy neighbors. We compare the poststarburst galaxies to currently star-forming galaxies identified solely by A-star excess or Hα emission. We find that poststarburst galaxies are in the same kinds of environments as star-forming galaxies; this is our null hypothesis. More importantly, we find that at each value of the A-star excess, the star-forming and poststarburst galaxies lie in very similar distributions of environment. Other studies finding similar results have argued that galaxy transformations occur slowly (timescales >1 Gyr), but this is at odds with the evidence that red galaxies are formed via starbursts. The only deviations from our null hypothesis are barely significant: a slight deficit of poststarburst galaxies (relative to the star-forming population) in very low-density regions, a small excess inside the virial radii of clusters, and a slight excess with nearby neighbors. None of these effects is strong enough to make the poststarburst galaxies a high-density phenomenon, or to argue that the starburst events are primarily triggered by external tidal impulses from close passages. The small excess inside cluster virial radii suggests that some poststarbursts are triggered by interactions with the intracluster medium, but this represents a very small fraction of all poststarburst galaxies.

The Kinematic Structure of Merger Remnants

We use numerical simulations to study the kinematic structure of remnants formed from mergers of equal-mass disk galaxies. In particular, we show that remnants of dissipational mergers, which include the radiative cooling of gas, star formation, feedback from supernovae, and the growth of supermassive black holes, are smaller, rounder, have, on average, a larger central velocity dispersion, and show significant rotation compared to remnants of dissipationless mergers. The increased rotation speed of dissipational remnants owes its origin to star formation that occurs in the central regions during the galaxy merger. We have further quantified the anisotropy, three-dimensional shape, minor-axis rotation, and isophotal shape of each merger remnant, finding that dissipational remnants are more isotropic, closer to oblate, have the majority of their rotation along their major axis, and are more disky than dissipationless remnants. Individual remnants display a wide variety of kinematic properties. A large fraction of the dissipational remnants are oblate isotropic rotators. Many dissipational remnants, and all of the dissipationless ones, are slowly rotating and anisotropic. The remnants of gas-rich major mergers can well reproduce the observed distribution of projected ellipticities, rotation parameter (V/?)*, kinematic misalignments, ?, and isophotal shapes. The dissipationless remnants are a poor match to this data. We also investigate the properties of merger remnants as a function of initial disk gas fraction, orbital angular momentum, and the mass of the progenitor galaxies. Our results support the merger hypothesis for the origin of low-luminosity elliptical galaxies provided that the progenitor disks are sufficiently gas-rich, however our remnants are a poor match to the bright ellipticals that are slowly rotating and uniformly boxy.

LUMINOSITY PROFILES OF dE AND dS0 GALAXIES IN THE VIRGO CLUSTER

We investigated the structural parameters of a sample of 30 dwarf galaxies(15 dEs and 15 dS0s) in the Virgo Cluster using i-band images from the Sloan Digital Sky Survey Data Release 4. Among 28 galaxies for which surface brightness profiles were derived from ellipse fittings, 23 galaxies had a single component that was adequately described by a generalized <TEX>$S\acute{e}rsic$</TEX> function with a shape parameter ranging from n=0.5 to 2, while 5 galaxies(2 dEs and 3 dS0s) had bulge and disk components that were fitted by a generalized <TEX>$S\acute{e}rsic$</TEX> function and an exponential function, respectively. Since the majority of dwarf galaxies in the present sample had a single component, it seems likely that genuine dS0 galaxies that have disk and bulge components are quite rare in the Virgo Cluster. The similarity in structural parameters of genuine dS0 galaxies in the Virgo Cluster with those of Magellanic-type galaxies implies that the progenitors of dwarf lenticular galaxies in the Virgo Cluster were most likely Magellanic-type galaxies if dS0s are harassed late-type spirals.

The Spiderweb Galaxy: A Forming Massive Cluster Galaxy at z ~ 2

We present a deep image of the radio galaxy MRC 1138-262 taken with the Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized structure, identified with a protocluster. The morphology shown on the new deep HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy features are observed, apparently star-forming satellite galaxies in the process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago. There is an extended emission component, implying that star formation was occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A striking feature of the newly named ``Spiderweb galaxy'' is the presence of several faint linear galaxies within the merging structure. The dense environments and fast galaxy motions at the centres of protoclusters may stimulate the formation of these structures, which dominate the faint resolved galaxy populations in the Hubble Ultra Deep Field. The new image provides a unique testbed for simulations of forming dominant cluster galaxies.

A Search for Low Surface Brightness Structure around Compact Narrow Emission Line Galaxies

As the most extreme members of the rapidly evolving faint blue galaxy population at intermediate redshift, the compact narrow emission line galaxies (CNELGs) are intrinsically luminous (-22 < M_B < -18) with narrow emission linewidths (30 < \sigma < 125 km/s). Their nature is heavily debated: they may be low-mass starbursting galaxies that will fade to present-day dwarf galaxies or bursts of star formation temporarily dominating the flux of more massive galaxies, possibly related to in situ bulge formation or the formation of cores of galaxies. We present deep, high-quality (~0.6 - 0.8 arcsec) images with CFHT of 27 CNELGs. One galaxy shows clear evidence for a tidal tail; the others are not unambiguously embedded in galactic disks. Approximately 55% of the CNELGS have sizes consistent with local dwarfs of small-to-intermediate sizes, while 45% have sizes consistent with large dwarfs or disks galaxies. At least 4 CNELGs cannot harbor substantial underlying disk material; they are low-luminosity galaxies at the present epoch (M_B > -18). Conversely, 15 are not blue enough to fade to low-luminosity dwarfs (M_B > -15.2). The majority of the CNELGs are consistent with progenitors of intermediate-luminosity dwarfs and low-luminosity spiral galaxies with small disks. CNELGs are a heterogeneous progenitor population with significant fractions (up to 44%) capable of fading into today's faint dwarfs (M_B > -15.2), while 15 to 85% may only experience an apparently extremely compact CNELG phase at intermediate redshift but remain more luminous galaxies at the present epoch.

Detecting reionization in the star formation histories of high-redshift galaxies

The reionization of cosmic hydrogen, left over from the big bang, increased its temperature to ≳104 K. This photoheating resulted in an increase of the minimum mass of galaxies and hence a suppression of the cosmic star formation rate (SFR). The affected population of dwarf galaxies included the progenitors of massive galaxies that formed later. We show that a massive galaxy at a redshift z≳ 6 should show a double-peaked star formation history marked by a clear break. This break reflects the suppression signature from reionization of the region in which the galaxy was assembled. Since massive galaxies originate in overdense regions where cosmic evolution is accelerated, their environment reionizes earlier than the rest of the universe. For a galaxy of ∼1012 M⊙ in stars at a redshift of z∼ 6.5, the SFR should typically be suppressed at a redshift z≳ 10 since the rest of the universe is known to have reionized by z≳ 6.5. Indeed, this is inferred to be the case for HUDF-JD2, a massive galaxy which is potentially at z∼ 6.5 but is inferred to have formed the bulk of its 3 × 1011 M⊙ in stars at z≳ 9.

Starburst Feedback in Local, Massive Galaxies

Major mergers of gas-rich galaxies, each comparable in mass to the Milky Way, are rare at the present epoch. These events were readily identifed, however, two decades ago in far-infrared sky surveys (Soifer et al. 1986, 1987). Removal of the dust enshrouding these starbursts was almost immediately proposed as an evolutionary path to quasar formation (Sanders 1988). Recent measurements of the stellar velocity dispersion, rotation speed, and stellar surface brightness profile of these mergers suggest ULIRGs are indeed progenitors of field elliptical galaxies (Genzel et al. 2001; Tacconi et al. 2002).

Timescales of merger, starburst and AGN activity in radio galaxy B2 0648+27

In this paper we use neutral hydrogen HI and optical spectroscopic observations to compare the timescales of a merger event, starburst episode and radio-AGN activity in the radio galaxy B2 0648+27. We detect a large ring-like structure of HI in emission around the early-type host galaxy of B2 0648+27 (M_HI = 8.5 x 10^9 Msun, diameter = 190 kpc). We interpret this as the result of a major merger that occurred > 1.5 Gyr ago. From modelling optical long-slit spectra we find that a young stellar population of 0.3 Gyr, indicative of a past starburst event, dominates the stellar light throughout the galaxy. The off-set in time between the merger event and the starburst activity in B2 0648+27 suggests that the starburst was triggered in an advanced stage of the merger, which can be explained if the gas-rich progenitor galaxies contained a bulge. Although the exact age of the radio source remains uncertain, there appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity. We also observe an outflow of emission-line gas in this system, which is likely related to superwinds driven by the stars that formed during the starburst event. We argue that the radio galaxy B2 0648+27 is a link in the evolutionary sequence between Ultra-Luminous Infrared Galaxies (ULIRGs) and genuine early-type galaxies.

The VIMOS-VLT Deep Survey

We have computed the evolution of the rest-frame B-band luminosity function (LF) for bulge and disk-dominated galaxies since z=1.2. We use a sample of 605 spectroscopic redshifts with I_{AB}<24 in the Chandra Deep Field South from the VIMOS-VLT Deep Survey, 3555 galaxies with photometric redshifts from the COMBO-17 multi-color data, coupled with multi-color HST/ACS images from the Great Observatories Origin Deep Survey. We split the sample in bulge- and disk-dominated populations on the basis of asymmetry and concentration parameters measured in the rest-frame B-band. We find that at z=0.4-0.8, the LF slope is significantly steeper for the disk-dominated population (\alpha=-1.19 \pm 0.07) compared to the bulge-dominated population (\alpha=-0.53 \pm 0.13). The LF of the bulge-dominated population is composed of two distinct populations separated in rest-frame color: 68% of red (B-I)_{AB}>0.9 and bright galaxies showing a strongly decreasing LF slope \alpha=+0.55 \pm 0.21, and 32% of blue (B-I)_{AB}<0.9 and more compact galaxies which populate the LF faint-end. We observe that red bulge-dominated galaxies are already well in place at z~1, but the volume density of this population is increasing by a factor 2.7 between z~1 and z~0.6. It may be related to the building-up of massive elliptical galaxies in the hierarchical scenario. In addition, we observe that the blue bulge-dominated population is dimming by 0.7 magnitude between z~1 and z~0.6. Galaxies in this faint and more compact population could possibly be the progenitors of the local dwarf spheroidal galaxies.

The dissipative merger progenitors of elliptical galaxies

We address the deviations of the scaling relations of elliptical galaxies from the expectations based on the virial theorem and homology, including the tilt of the fundamental plane and the steep decline of density with mass. We show that such tilts result from dissipative major mergers once the gas fraction available for dissipation declines with progenitor mass, and derive the scaling properties of the progenitors. We use hydrodynamical simulations to quantify the effects of major mergers with different gas fractions on the structural properties of galaxies. The tilts are driven by the differential shrinkage of the effective stellar radius as a function of dissipation in the merger, while the correlated smaller enhancements in internal velocity and stellar mass keep the slope of the velocity-stellar mass relation near V \pr M_*^{1/4}. The progenitors match a straightforward model of disc formation in LCDM haloes. Their total to stellar mass ratio within the effective radius varies as M/M_* \pr M_*^-0.25, consistent with supernova feedback. They are nearly homologous in the sense that the dark-matter fractions within the effective and virial radii scale with mass in a similar way, with only a weak decline of density with mass. The progenitor radius is roughly R \pr M_*^0.3, compatible with today's intermediate late-type galaxies. This indicates that the latest wet mergers involved relatively big discs. The dissipative gas to stellar mass ratio is predicted to decline \pr M_*^-0.5, similar to the observed trend in today's blue sequence galaxies. Such a trend should be observed in relatively massive gaseous galaxies at z~1-4. The corresponding baryonic relations are consistent with homology and spherical virial equilibrium, V \pr R \pr M_b^1/3.

The clustering evolution of distant red galaxies in the GOODS-MUSIC sample

Aims.We study the clustering properties of Distant Red Galaxies (DRGs) to test whether they are the progenitors of local massive galaxies.

The effect of gravitational recoil on black holes forming in a hierarchical universe

Galactic bulges are known to harbour central black holes whose mass is tightly correlated with the stellar mass and velocity dispersion of the bulge. In a hierarchical universe, galaxies are built up through successive mergers of subgalactic units, a process that is accompanied by the amalgamation of bulges and the likely coalescence of galactocentric black holes. In these mergers, the beaming of gravitational radiation during the plunge phase of the black hole collision can impart a linear momentum kick or ‘gravitational recoil’ to the remnant. If large enough, this kick will eject the remnant from the galaxy entirely, and populate intergalactic space with wandering black holes. Using a semi-analytic model of galaxy formation, we investigate the effect of black hole ejections on the scatter of the relation between black hole and bulge mass. We find that while not being the dominant source of the measured scatter, they do provide a significant contribution and may be used to set a constraint, vkick≲ 500 km s−1, on the typical kick velocity, in agreement with values found from general relativistic calculations. Even for the more modest kick velocities implied by these calculations, we find that a substantial number of central black holes are ejected from the progenitors of present-day galaxies, giving rise to a population of wandering intrahalo and intergalactic black holes whose distribution we investigate in high-resolution N-body simulations of the Milk Way mass haloes. We find that intergalactic black holes make up only ∼2–3 per cent of the total galactic black hole mass but, within a halo, wandering black holes can contribute up to about half of the total black hole mass orbiting the central galaxy. Intrahalo black holes offer a natural explanation for the compact X-ray sources often seen near the centres of galaxies and for the hyperluminous non-central X-ray source in M82.

The Fundamental Scaling Relations of Elliptical Galaxies

We examine the fundamental scaling relations of elliptical galaxies formed through mergers. Using hundreds of simulations to judge the impact of progenitor galaxy properties on the properties of merger remnants, we find that gas dissipation provides an important contribution to tilt in the Fundamental Plane relation. Dissipationless mergers of disks produce remnants that occupy a plane similar to that delineated by the virial relation. As the gas content of progenitor disk galaxies is increased, the ti lt of the resulting Fundamental Plane relation increases and the slope of the Re - M⋆ relation steepens. For gas fractions fgas > 30%, the simulated Fundamental Plane scalings (Re ∝ σ 1.55 I -0.82 e ) approach those observed in the K-band (Re ∝ σ 1.53 I -0.79 e ). The dissipationless merging of spheroidal galaxies and the re-merging of disk galaxy remnants roughly maintain the tilt of the Fundamental Plane occupied by the progenitor ellipticals, approximately independent of the orbital energy or angular momentum. Dry merging of spheroidal systems at redshifts z < 1 is then expected to maintain the stellar-mass Fundamental Plane relations imprinted by gas-rich merging during the epoch of rapid spheroid and supermassive black hole growth at redshifts z ≈ 1 - 3. In our simulations, feedback from supermassive black hole growth has only a minor influence on the stellar-mass sca ling relations of spheroidal galaxies, but may play a role in maintaining the observed Fundamental Plane tilt at optical wavelengths by suppressing residual star formation in merger remnants. We estimate that ≈ 40 - 100% of the Fundamental Plane tilt induced by structural properties, as opposed to stellar population effects, owes to trends in the central total-to-stellar mass ratio Mtotal/M⋆ produced by dissipation. Gas cooling allows for an increase in central s tellar phase-space density relative to dissipationless mergers, thereby decreasing the central Mtotal/M⋆. Lower mass systems obtain greater phase-space densities than higher mass systems, producing a galaxy mass-dependent central Mtotal/M⋆ and a corresponding tilt in the Fundamental Plane. We account for these trends in the importance of dissipation with galaxy mass in terms of the inefficient cooling of collisionally heated gas in massi ve halos and dynamically varying gas consumption timescales in smaller systems.

Merging of Elliptical Galaxies as a Possible Origin of the Intergalactic Stellar Population

We present N-body simulations of elliptical galaxy encounters into dry mergers to study the resulting unbound intergalactic stellar population, in particular that of the post-main-sequence stars. The systems studied are pairs of spherical galaxies without dark halos. The stellar content of the model galaxies is distributed into mass bins representing low- and intermediate-mass stars (0.85-8 M☉) according to the Salpeter initial mass function. Our models follow the dynamical evolution of galaxy encounters colliding head-on from initial low-energy parabolic or high-energy mildly hyperbolic orbits and for a choice of initial mass ratios. The merging models with initial parabolic orbits have M2/M1 = 1 and 10, and they leave behind, respectively, 5.5% and 10% of the total initial mass as unbound stellar mass. The merging model with an initial hyperbolic orbit has M2/M1 = 1 and leaves behind 21% of its initial stellar mass as unbound mass, showing that the efficiency in producing intergalactic stars through a high-energy hyperbolic encounter is about 4 times that of a parabolic encounter of the same initial mass ratio. By assuming that all progenitor galaxies, as well as the merger remnants, are homologous systems we find that the intergalactic starlight is 17% and 28% of the total starlight, respectively, for the parabolic and hyperbolic encounters with M2/M1 = 1. In all models, stars of different mass have the same probability of becoming unbound and feeding the intergalactic stellar population.

The Las Campanas/Anglo-Australian Telescope Rich Cluster Survey — III. Spectroscopic studies of X-ray bright galaxy clusters at z~ 0.1

We present the analysis of the spectroscopic and photometric catalogues of 11 X-ray luminous clusters at 0.07 < z < 0.16 from the Las Campanas/Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic data set consists of over 1600 galaxy cluster members, of which two-thirds are outside r(200). These spectra allow us to assign cluster membership using a detailed mass model and expand on our previous work on the cluster colour-magnitude relation ( CMR) where membership was inferred statistically. We confirm that the modal colours of galaxies on the CMR become progressively bluer with increasing radius d( B - R)/dr(p) = - 0.011 +/- 0.003 and with decreasing local galaxy density d( B - R)/dlog ( Sigma)= - 0.062 +/- 0.009. Interpreted as an age effect, we hypothesize that these trends in galaxy colour should be reflected in mean H delta equivalent width. We confirm that passive galaxies in the cluster increase in Hd line strength as dH delta/dr(p) = 0.35 +/- 0.06. Therefore, those galaxies in the cluster outskirts may have younger luminosity-weighted stellar populations; up to 3 Gyr younger than those in the cluster centre assuming d( B - R)/dt = 0.03 mag per Gyr. A variation of star formation rate, as measured by [ O II]lambda 3727 angstrom, with increasing local density of the environment is discernible and is shown to be in broad agreement with previous studies from the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey. We divide our spectra into a variety of types based upon the MORPHs classification scheme. We find that clusters at z similar to 0.1 are less active than their higher-redshift analogues: about 60 per cent of the cluster galaxy population is non-star forming, with a further 20 per cent in the post-starburst class and 20 per cent in the currently active class, demonstrating that evolution is visible within the past 2 - 3 Gyr. We also investigate unusual populations of blue and very red non-star forming galaxies and we suggest that the former are likely to be the progenitors of galaxies which will lie on the CMR, while the colours of the latter possibly reflect dust reddening. We show that the cluster galaxies at large radii consist of both backsplash ones and those that are infalling to the cluster for the first time. We make a comparison to the field population at z similar to 0.1 and examine the broad differences between the two populations. Individually, the clusters show significant variation in their galaxy populations which we suggest reflects their recent infall histories.

Virial Masses and the Baryon Fraction in Galaxies

We have measured the weak lensing signal as a function of restframe luminosity for a sample of `isolated' galaxies. These results are based on four-band photometry from the Red-Sequence Cluster Survey, enabling us to determine photometric redshifts for a large number of galaxies. We select a secure sample of lenses with photometric redshifts 0.2<z<0.4 and study the relation between the virial mass and baryonic contents. In addition, we discuss the implications of the derived photometric redshift distribution for published cosmic shear studies. The virial masses are derived from a fit to the observed lensing signal. For a galaxy with a fiducial luminosity of 10^10 h^-2 L_Bsun we obtain a mass M_vir=9.9^{+1.5}_{-1.3}\times 10^11 h^-1 M_sun. The virial mass as a function of luminosity is consistent with a power-law ~L^1.5, with similar slopes for the three filters considered here. These findings are in excellent agreement with results from the Sloan Digital Sky Survey and semi-analytic models of galaxy formation. We measure the fraction of mass in stars and the baryon fraction in galaxies by comparing the virial mass-to-light ratio to predicted stellar mass-to-light ratios. We find that star formation is inefficient in converting baryons into stars, with late-type galaxies converting ~33% and early-type galaxies converting only ~14% of baryons into stars. Our results imply that the progenitors of early-type galaxies must have low stellar mass fractions, suggestive of a high formation redshift.

Morphological evolution of discs in clusters

The recent discovery of hidden non-axysimmetric and disk-like structures in bright Virgo dwarf elliptical and lenticular galaxies (dE/dSph/dS0) indicates that they may have late-type progenitors. Using N-body simulations we follow the evolution of disk galaxies within aCDM cluster simulated with 10 7 particles, where the hierarchical growth and galaxy harassment are modeled self-consistently. Most of the galaxies undergo significant morphological transformation, even at the outskirts of the cluster, and move through the Hubble sequence from late type disks to dwarf spheroidals. None of the disks are completely destroyed therefore they can not be the progenitors of ultra compact dwarf galaxies (UCDs). The time evolution of the simulated galaxies is compared with unsharp-masked images obtained from VLT data and the projected kinematics of our models with the latest high resolution spectroscopic studies from the Keck and Palomar telescopes.